Laser-based spectroscopic instruments have been implemented in a variety of environments to extract measurement data. Laser-based measurement apparatus can be implemented in situ and offer the further advantage of high speed feedback suitable for dynamic process control. One technique for measuring combustion species such as gas composition, temperature and other combustion parameters (collectively, “combustion properties”) utilizes Tunable Diode Laser Absorption Spectroscopy (TDLAS). TDLAS is typically implemented with diode lasers operating in the near-infrared and mid-infrared spectral regions. Suitable lasers have been extensively developed for use in the telecommunications industry and are, therefore, readily available for TDLAS. Various techniques for TDLAS which are more or less suitable for sensing control of combustion processes have been developed. Commonly known techniques are wavelength modulation spectroscopy and direct absorption spectroscopy. Each of these techniques is based upon a predetermined relationship between the quantity and nature of laser light received by a detector after the light has been transmitted through a combustion zone (or combustion chamber) and absorbed in specific spectral bands which are characteristic of the combustion species present in the combustion zone. The absorption spectrum received by the detector is used to determine the combustion properties, including the quantity of the combustion species under analysis and associated combustion parameters such as temperature.
One particularly useful implementation of TDLAS utilizes wavelength-multiplexed diode laser measurements in order to monitor multiple combustion species and combustion parameters. One such system is described in PCT/US2004/010048 (International Publication No. WO 2004/090496) entitled “Method and Apparatus for the Monitoring and Control of Combustion” (“WO '496”), the content of which is incorporated in its entirely herein.
Determining combustion properties can be used to improve combustion efficiency in, for example, gas turbine engines, while simultaneously reducing the harmful emissions such as nitrogen oxides. Monitoring combustion properties within gas turbine engines also has the potential to improve turbine blade lifetime and all other engine components aft of the combustion zone as well as providing a useful diagnostic to identify malfunctioning engines.
While monitoring combustion properties in gas turbine engines would appear to have many potential benefits, making the measurements has proven extremely difficult. The difficulty stems from two major sources. First, the high-pressure and temperature of the combustion zone (30-40 bar, 2200 K) creates an environment in which normal spectral features are highly distorted, leading to difficulty in interpreting data even if it can be obtained. Second, making such measurements in an operating engine requires optical access; that is, a penetration or penetrations in the engine casing through which one can direct a laser beam over a line of sight. This is very difficult to arrange in an operating gas turbine engines due to the harsh nature of the engine environment, the limited space available for monitoring components and the need to minimize impact on critical components.
To illustrate the difficulty of providing line of sight optical access to the combustion zone of a gas turbine engine, FIG. 1 is a schematic view of a gas turbine engine 10 including a combustion zone 12. The combustion zone 12 is defined between a cylindrical outer casing 14 and a cylindrical inner casing 16. A turbine shaft 18 resides within the inner casing 16. The confined area in the vicinity of the combustion zone complicates effective access.
FIG. 2 is a schematic cross-section of the combustion zone 12 taken along lines A-A of FIG. 1. FIG. 2 shows the cylindrical outer casing 14, the cylindrical inner casing 16, the turbine shaft 18 and a number of combustor fuel cups 20 between the inner and outer casings. One possibility for providing line of sight access to the combustion zone is to provide a transmitting optic 22 associated with the borescope port 24 on the outer casing and a receiving optic 26 associated with a port in the inner casing. However, the turbine shaft that is housed in the inner casing prevents any optics from being placed inside the inner casing.
A second possibility is illustrated in FIG. 3, with like reference numbers associated with like elements. Here a line of sight is provided by passing the laser from one borescope inspection port 24A to a second borescope inspection port 24B. In such an embodiment, the line of sight skirts the central inner casing essentially forming a cord 28 through the annular combustion space. While potentially feasible, such a design is problematic because of the high-pressure, high-temperature environment and the difficulty of steering the beam at the severe angle required by the engine geometry.
The present invention is directed toward overcoming one or more of the problems discussed above.